1. Field of the Invention
The present invention relates to a voltage-controlled current source. More particularly, the present invention relates to a voltage-controlled current source and a frequency scanner using the voltage-controlled current source.
2. Description of Related Art
Many electronic and communication equipments use current source. In the design of analog circuits, it is difficult to design a good current source in circuit design, particularly when the change of load is great or a certain mechanism is required to control the current output by the current source.
FIG. 1 is a circuit diagram of a conventional operational transconductance amplifier (OTA) voltage-controlled current source disclosed in U.S. Pat. No. 5,815,039. Referring to FIG. 1, the circuit includes BJT transistors Q1˜Q8 and two current sources IO. The circuit mainly uses differential input voltages (½ΔVi and−½ΔVi as shown in FIG. 1), i.e. differences between the base voltages of BJT transistors Q1 and Q5 (or BJT transistors Q2 and Q4) to control the magnitude of differential output current ΔI. However, it is known that the conventional OTA has disadvantages of being only applicable to small signals and outputting small current ΔI, and thus the operating scope of the input voltage (½ΔVi and−½ΔVi as shown in FIG. 1) of the circuit to the output current ΔI is quite small, and only the small signals can be processed.
FIG. 2 is a circuit diagram of a conventional switch current source. The circuit includes resistors R1˜RN, switches S21˜S2N, an amplifier A201, and a transistor Q201. Mainly, a turn-on state or a turn-off state of the switches S21˜S2N is used to control the impedance between a common voltage VDD to an emitter of the transistor Q201, so as to control the current ΔI flowing through the transistor Q201. However, the current source using the switches S21˜S2N to control the impedance cannot realize a continuous current control, but provide an intermittent adjustment.